Laser irradiation apparatus and laser irradiation method

ABSTRACT

It is an object of the present invention to provide a laser irradiation apparatus which can manufacture a homogenously crystallized film by varying the energy intensity of an irradiation beam in forward and backward directions of the irradiation. A laser irradiation apparatus of the present invention comprises a laser oscillator and means for varying beam intensity wherein a laser beam is obliquely incident into the irradiation surface, the laser beam is scanned relative to the irradiation surface, and the beam intensity is varied in accordance with the scanning direction. Further, the laser oscillator is a continuous wave solid-state laser, gas laser, or metal laser. A pulsed laser having a repetition frequency of 10 MHz or more can also be used.

TECHNICAL FIELD

The present invention relates to a laser irradiation apparatus (anapparatus including a laser and an optical system to lead a laser beamoutputted from the laser to an irradiation body) and a laser irradiationmethod for annealing, for example, a semiconductor material effectivelyand homogeneously.

BACKGROUND ART

In recent years, a technique to manufacture a thin film transistor(hereinafter referred to as a TFT) over a substrate has made greatprogress and application to an active matrix display device has beenadvanced. In particular, since a TFT using a poly-crystallinesemiconductor film has higher field-effect mobility than a conventionalTFT using an amorphous semiconductor film, high-speed operation hasbecome possible. Therefore, it has been tried to control a pixel by adriver circuit formed over the same substrate as the pixel, which hasbeen conventionally provided outside the substrate.

With the increase of the demand for semiconductor devices, it has beenrequired to manufacture the semiconductor devices at lower temperaturein shorter time. A glass substrate, which is superior to a quartzsubstrate in terms of cost, has been employed as a substrate for asemiconductor device. In the case of forming a TFT with apoly-crystalline semiconductor film over a glass substrate, although aglass substrate is sensitive to heat and easy to deform due to the heat,a semiconductor film can be easily crystallized at low temperature byemploying laser annealing.

In addition, compared with another annealing method which uses radiantheat or conductive heat, the laser annealing has advantages that theprocessing time can be shortened drastically and that a semiconductorfilm over a substrate can be heated selectively and locally so thatalmost no thermal damage is given to the substrate.

As laser oscillators used in the laser annealing, there are pulsed laseroscillators and continuous wave (CW) laser oscillators according totheir oscillation methods. An excimer laser has advantages of highoutput power and capability of repetitive irradiation at high repetitionfrequency. Moreover, a laser beam emitted from the excimer laser has anadvantage of high absorption coefficient to a silicon film, which isoften used as a semiconductor film. It is preferable to perform thelaser irradiation in such a way that a laser beam is transformed into arectangular, linear, or elliptical shape on an irradiation surface by anoptical system and then the beam is scanned relative to the irradiationsurface in a minor-axis direction of the rectangular, linear, orelliptical beam, because this method provides high productivity and issuperior industrially. At present, a liquid crystal display and an EL(electroluminescent) display are often manufactured by forming TFTs witha semiconductor film crystallized according to this technique.

On the other hand, when a laser beam emitted from a continuous wavelaser (the laser beam is hereinafter referred to as a CW laser beam) istransformed into a rectangular, linear, or elliptical shape and thesubstrate is moved relatively in the minor-axis direction of therectangular, linear, or elliptical beam, a large grain crystal extendinglong in the moving direction can be formed. In the case of manufacturinga TFT in accordance with the major-axis direction of the large graincrystal, the TFT has higher mobility than a TFT manufactured with theexcimer laser. Since a circuit can be driven at high speed by using theTFT formed with the CW laser beam, a driver circuit for driving adisplay, a CPU, and the like can be manufactured.

Conventionally, a laser irradiation apparatus shown in FIG. 8 has beenknown. This laser irradiation apparatus comprises a plurality ofcylindrical lens arrays and the like. A laser beam emitted from a laseroscillator 1 is divided into a plurality of beams using a plurality ofcylindrical lens arrays 2 to 6 and condensed. Then, after the laserbeams are reflected on a mirror 7, the laser beams are condensed intoone rectangular, linear, or elliptical laser beam with a doubletcylindrical lens 8 consisting of two cylindrical lenses. After that, thelaser beam is delivered vertically to an irradiation surface 9. Bydelivering the rectangular, linear, or elliptical beam to theirradiation surface while moving the beam relatively in the minor-axisdirection of the linear beam, the whole surface of an amorphoussemiconductor can be annealed so that the amorphous semiconductor iscrystallized, the crystallinity thereof is enhanced, or an impurityelement is activated.

However, since the conventional laser irradiation apparatus needs to usea plurality of expensive cylindrical lens arrays and to arrange them soas to form a desired rectangular, linear, or elliptical beam asdescribed above, the apparatus has a problem in that the size and costthereof increase. Further, since the laser beam, which has been shapedinto a rectangular, linear, or elliptical spot is delivered verticallyto the irradiation surface, that is, a surface of a semiconductor filmformed over a substrate, the beam being incident from above thesemiconductor film passes through the substrate and is reflected at abottom surface of the substrate. Then, the beam incident from aboveinterferes with the beam reflected at the bottom surface. Thus,sometimes a homogeneous crystalline semiconductor film cannot bemanufactured.

The present applicant has already suggested a compact and inexpensivelaser irradiation apparatus which has overcome the problems of theconventional laser irradiation apparatus. The laser irradiationapparatus is illustrated in FIG. 9. This laser irradiation apparatususes a convex lens 13 into which the laser beam is incident obliquely sothat the laser beam is extended to form a rectangular, linear, orelliptical beam 14. Then, the extended beam is delivered to anirradiation surface 15 obliquely.

That is to say, this laser irradiation apparatus comprises a laseroscillator 11, a mirror 12, the convex lens 13, and the like. A laserbeam emitted from the laser oscillator 11 is reflected on a mirror 12and incident obliquely into the convex lens 13 so that the laser beam isshaped into the rectangular, linear, or elliptical beam 14. The beam 14is delivered to the irradiation surface 15. With this structure, theapparatus can be made small, and the adverse effect due to theinterference caused by the reflected beam from the bottom surface of thesubstrate can be prevented (see Reference 1: Japanese Patent ApplicationLaid-Open No. 2003-257885).

However, the above laser irradiation apparatus still has the followingproblem. Although laser annealing is performed with a CW laser beam, forexample with a CW laser beam providing 10 W at 532 nm having arectangular shape of 300 μm in its major-axis direction and 10 μm in itsminor-axis direction, the width of the large grain crystal formed by onescanning is as narrow as approximately 200 μm. For this reason, in orderto crystallize the whole surface of the substrate effectively by thelaser annealing, the laser beam needs to be scanned back and forth whiledisplacing the laser beam by the width of the large grain crystal formedby one scanning of the beam. At this time, if the intensity distributionof the laser beam in the minor-axis direction is not symmetric along aplane which passes through the center of the minor axis of the beam,which is perpendicular to the substrate, and which is parallel to themajor axis of the beam, the crystallization state after the laserannealing may be different between back scanning and forth scanning.

However, when the rectangular, linear, or elliptical beam is deliveredobliquely to the irradiation surface and the substrate is moved in theminor-axis direction of the beam, the state of the laser beam deliveredto the amorphous semiconductor film is different according to thescanning direction of the laser beam as described later. Thus,homogeneous crystallization is difficult to be performed.

DISCLOSURE OF INVENTION

The present invention is to solve the above problems, and specifically,it is an object of the present invention to provide a laser irradiationapparatus and a laser irradiation method which can manufacture ahomogeneously crystallized film by varying the energy density of anirradiation beam according to forward and backward scanning directions.

To achieve the above object, the present invention employs aconstitution below. It is to be noted that the laser annealing methodherein described indicates a technique to crystallize an amorphousregion or a damaged region formed by adding ions to a semiconductorsubstrate or a semiconductor film, a technique to crystallize asemiconductor film by irradiating an amorphous semiconductor film formedover a substrate with a laser beam, a technique to crystallize asemiconductor film which is not single crystal (the above semiconductorfilms which are not single crystals are generically referred to as anon-single crystal semiconductor film) by laser irradiation afterintroducing an element for promoting crystallization such as nickel intothe semiconductor film, and so on. Moreover, the laser annealingincludes a technique applied to planarization or modification of thesurface of a semiconductor substrate or a semiconductor film.

According to an aspect of the present invention disclosed in thisspecification, a laser irradiation apparatus comprises a laseroscillator, means for varying beam intensity, and a convex lens or adiffractive optical element, wherein a laser beam is incident into anirradiation surface, wherein the laser beam is scanned relative to theirradiation surface, and wherein the means for varying beam intensityvaries the beam intensity in every scanning direction.

According to another aspect of the present invention, a laserirradiation apparatus comprises a laser oscillator, means for varyingbeam intensity, and a convex lens or a diffractive optical element,wherein a laser beam is incident into an irradiation surface, whereinthe laser beam is scanned relative to the irradiation surface, whereinthe means for varying beam intensity varies the beam intensity in everyscanning direction, and wherein the crystallization state after thelaser annealing at the irradiation surface is homogeneous in all thescanning directions.

In the above constitution, the laser beam may be incident obliquely intothe irradiation surface.

According to another aspect of the present invention, the laser beampassed through the convex lens or the diffractive optical element has arectangular, linear, or elliptical shape on the irradiation surface.

According to another aspect of the present invention, the means forvarying beam intensity is a polarizing plate. The number of thepolarizing plates may be more than one.

According to another aspect of the present invention, the laseroscillator is a CW solid-state laser, gas laser, or metal laser, or apulsed solid-state laser, gas laser, or metal laser. As the solid-statelaser, there are a YAG laser, a YVO₄ laser, a YLF laser, a YAlO₃ laser,a GdVO₄ laser, a Y₂O₃ laser, a glass laser, a ruby laser, an alexandritelaser, a Ti:sapphire laser, and the like. As the gas laser, there are anAr laser, a Kr laser, a CO₂ laser, and the like. As the metal laser,there are a copper vapor laser, a gold vapor laser, and the like. As thepulsed laser, a YVO₄ laser, a GdVO₄ laser, a YAG laser, and the likewhich have a repetition frequency of 10 MHz or more can be used.

According to another aspect of the present invention, a laser beamemitted from the laser oscillator is converted into a harmonic by anon-linear optical element.

According to another aspect of the present invention, a laser beam isemitted from a laser oscillator, the laser beam passes through means forvarying beam intensity which can vary the beam intensity according to ascanning direction of the laser beam, the laser beam passes through aconvex lens or a diffractive optical element, and then the laser beam isincident into an irradiation surface. Further, by scanning the laserbeam relative to the irradiation surface, the irradiation surface can beannealed equally in all the scanning directions. The laser beam may beincident obliquely into the irradiation surface.

According to the present invention, since the laser beam is deliveredobliquely to the irradiation surface, the interference between theincident beam and the reflected beam from the bottom surface of thesubstrate can be suppressed and a laser beam with homogenous energydistribution can be delivered to the irradiation surface. Thus, anon-single crystal semiconductor film over a substrate can be annealedhomogeneously.

Next, the beam intensity is changed by a polarizing plate or the like,which is the means for varying beam intensity, in accordance with thescanning direction. Specifically, the laser beam is scanned on the wholeirradiation surface as single-stroke drawing while changing the scanningdirection in the forward or backward direction. Thus, the wholenon-single crystal semiconductor film over a substrate can becrystallized homogeneously. With the CW laser or the pulsed laser havinga repetition frequency of 10 MHz or more, a large grain crystal can bemanufactured; therefore, a TFT having high mobility can be manufactured.

For this reason, the variation in the electrical characteristic can bedecreased, and the reliability can be enhanced. By applying the presentinvention to a mass-production line of TFTs, TFTs having high operatingcharacteristics can be produced effectively. As a result, asemiconductor device, typically an active matrix type liquid crystaldisplay device and an active matrix EL display device, having highoperating characteristic and high reliability can be achieved. Further,in a manufacturing process of the semiconductor device, margin can beexpanded and the yield can be boosted; therefore manufacturing cost ofthe semiconductor device can be decreased.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 schematically shows a laser irradiation apparatus according tothe present invention;

FIG. 2 schematically shows another laser irradiation apparatus accordingto the present invention;

FIG. 3 is a graph showing the relation between the energy density andthe electron mobility of a TFT;

FIG. 4 is a graph showing the distribution of the electron mobility of aTFT;

FIG. 5 shows the state of a plane of a substrate;

FIG. 6 is a cross-sectional view taken along a line A-A′ of FIG. 5;

FIG. 7 is a cross-sectional view taken along a line B-B′ of FIG. 5;

FIG. 8 schematically shows a conventional laser irradiation apparatus;

FIG. 9 schematically shows another conventional laser irradiationapparatus;

FIGS. 10A to 10D schematically show a process of manufacturing a TFTaccording to the present invention; and

FIGS. 11A to 11C schematically show electronic appliances according tothe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 shows an example of a laser irradiation apparatus. First, asubstrate 107 with a non-single crystal semiconductor film 106 formed isprepared. The substrate 107 is set over an X-axis stage 108 and a Y-axisstage 109. The substrate 107 can be moved freely in X-axis and Y-axisdirections by moving the X-axis stage 108 and the Y-axis stage 109 indirections indicated with arrows respectively by a motor which is notshown. The X-axis stage 108 is scanned in forward and backwarddirections indicated with arrows.

The laser irradiation apparatus comprises a laser oscillator 101, twopolarizing plates 102 and 103, a mirror 104, and a convex lens 105. Thelaser oscillator 101 is a CW laser oscillator. As a CW solid-statelaser, there are a YAG laser, a YVO₄ laser, a YLF laser, a YAlO₃ laser,a GdVO₄ laser, a Y₂O₃ laser, an alexandrite laser, and a Ti:sapphirelaser. As a CW gas laser, there are an Ar laser, a Kr laser, and a CO₂laser. Not only the CW laser but also a pulsed laser can be used. As thepulsed laser, a YVO₄ laser, a GdVO₄ laser, a YAG laser, or the likewhich has a repetition frequency of 10 MHz or more can be used.

A laser beam emitted from the laser oscillator 101 is desirablyconverted into a harmonic by a non-linear optical element. For example,it is known that the YAG laser emits a laser beam with a wavelength of1065 nm. The absorption coefficient of this laser beam to a silicon filmis very low, and it is technically difficult to crystallize an amorphoussilicon film, which is one of semiconductor films, with this laser beam.However, by using a non-linear optical element, the laser beam can beconverted into a shorter wavelength. As the harmonic, the secondharmonic (532 nm), the third harmonic (355 nm), the fourth harmonic (266nm), and the fifth harmonic (213 nm) are given. Since these harmonicshave high absorption coefficient to an amorphous silicon film, theharmonics can be used for crystallizing the amorphous silicon film.

The polarizing plate 102 (103) corresponds to the means for varying beamintensity according to the present invention. The polarizing plate is acomponent which passes only light vibrating in a certain direction amongthe light vibrating in all directions of 360° and which blocks the lightvibrating in the other directions except the certain direction.Specifically, the intensity of a laser beam emitted from the laseroscillator 101 can be changed as desired by adjusting the angles betweentransmission axes of the two polarizing plates 102 and 103. In the caseof using such means for varying beam intensity, it is preferable to usea laser beam with the polarization ratio of 100:1 or more. This makes itpossible for the polarizing plate to adjust the energy within the rangeof approximately 0 to 99%.

Specifically, for example, the polarizing plate 102 is fixed, and thepolarizing plate 103 is set so that the polarizing plate 103 can freelyrotate either in a positive or negative direction. Then, by rotating thepolarizing plate 103 at a predetermined angle in the positive ornegative direction, the intensity of the laser beam which has passedthrough the two polarizing plates 102 and 103 and travels to the mirror104 can be changed. By changing the rotating angle as desired, theintensity of the laser beam can be changed to be any intensity. It is tobe noted that the number of the polarizing plates may be one, or morethan two.

When a laser beam obliquely enters the convex lens 105 from above, theincident beam having an approximately circular shape is extended long ina direction perpendicular to the direction indicated with the arrow,which is the moving direction of the X-axis stage 108, due to theastigmatism of the lens at a position where a focal line is formed.Then, a beam 120 on the irradiation surface has a rectangular, linear,or elliptical shape with the minor-axis length W and the major-axislength H. Since focal lines can be formed at two locations, whichevermay be chosen. However, because the two focal lines intersect with eachother, a practitioner needs to select appropriately. For the details,the reference 1 may be referred to.

In this case, the major-axis beam length H can be made longer byshortening the minor-axis beam length W. It is preferable to scan thebeam 120 in its minor-axis direction because the region where the laserbeam can be scanned at one time can be enlarged, which increases theproductivity.

A procedure for annealing the non-single crystal semiconductor film 106with the laser irradiation apparatus is described. First, by moving theX-axis stage 108 and the Y-axis stage 109, the substrate 107 is moved toa position where the annealing starts. Then, a CW laser beam is emittedfrom the laser oscillator 101, and the beam intensity of the CW laserbeam is adjusted by the polarizing plates 102 and 103 to be as high aspossible but within the range where the film is not peeled (thenon-single crystal semiconductor film is not ablated). The adjusted beamhaving an approximately circular shape is reflected on the mirror 104,and then enters obliquely the apex of the convex lens 105. The beam iselongated with the convex lens 105 so that the beam has a rectangular,linear, or elliptical shape with the minor-axis length W in the movingdirection indicated with an arrow of the X-axis stage 108 and themajor-axis length H in the direction perpendicular to the movingdirection. Then, the beam is delivered obliquely to the non-singlecrystal semiconductor film 106.

Then, the X-axis stage 108 is moved in the forward direction, which isthe same direction as the direction where the laser beam is incident, toanneal the non-single crystal semiconductor film 106. The X-axis stage108, when having reached the end of the forward direction, stops tomove, and then the Y-axis stage 109 is moved in the Y-axis direction byapproximately the length corresponding to the major-axis length H.

Subsequently, one of the polarizing plates 102 and 103 is rotated by apredetermined angle in the positive or negative direction to decreasethe beam intensity. At the same time, the X-axis stage 108 is moved in abackward direction, which is the direction toward the direction wherethe laser beam is incident, to anneal the non-single crystalsemiconductor film 106. The X-axis stage 108, when having reached theend in the backward direction, stops to move, and then the Y-axis stage109 is moved in the Y-axis direction by approximately the lengthcorresponding to the major-axis length H.

Subsequently, one of the polarizing plates 102 and 103 is rotated by apredetermined angle in the positive or negative direction, and the beamintensity is increased to be the original high beam intensity. Then, theX-axis stage 108 is moved in the forward direction, which is the samedirection as the direction where the laser beam is incident. After that,by repeating the above operation, the whole surface of the non-singlecrystal semiconductor film 106 can be annealed continuously assingle-stroke drawing so that the non-single semiconductor film 106 iscrystallized. Although the whole surface of the substrate can beannealed by scanning the beam in one of the forward and backwarddirections without changing the beam intensity, this annealing method islow in productivity, because the throughput by this method is a half ofthat by the annealing method performed while scanning the beam in bothforward and backward directions.

By annealing the whole surface of the non-single crystal semiconductorfilm 106 according to this procedure, the whole surface thereof can becrystallized homogeneously.

Next, the reason why the beam intensity has to be changed by thepolarizing plate 102 (103) is described. According to the experiments ofthe present inventors, it has been confirmed that the laser beam havinghigher energy density can manufacture a TFT having higher mobility. FIG.3 is a graph showing the result of the experiment on the average valueof the electron mobility of an n-ch TFT and the energy density of thelaser beam. The horizontal axis shows the proportion of the energydensity of the laser beam when it is assumed that the threshold of theenergy density at which the semiconductor film is ablated is 100%, andthe vertical axis shows the average value of the electron mobility ofthe manufactured TFTs. It is understood from FIG. 3 that the TFT havinghigher electron mobility can be manufactured when the semiconductor filmis annealed with the laser beam having higher energy density within therange of the threshold or less.

However, the annealing with a laser beam having the same beam intensityin the forward and backward directions using the laser irradiationapparatus shown in FIG. 1 has the problem shown in FIG. 4. FIG. 4 showsthe result of the experiment in the case of annealing with the laserbeam having the same beam intensity in the forward and backwarddirections using the laser irradiation apparatus shown in FIG. 1. InFIG. 4, the horizontal axis shows the major-axis direction of arectangular beam, while the vertical axis shows the electron mobility ofthe manufactured TFT.

As this result shows, in the case of irradiating the irradiation surfaceobliquely with a laser beam by the laser irradiation apparatus, the TFTmanufactured by scanning the laser beam in the forward directiongenerally has approximately 30% lower electron mobility than the TFTmanufactured by scanning the laser beam in the backward direction. It isnot preferable to manufacture TFTs using a semiconductor film havingsuch variation.

The reason why the result shown in FIG. 4 is brought is described withreference to FIGS. 5 to 7. FIG. 5 is a plain view of the substrate whichis seen from above. FIG. 6 is a cross-sectional view taken along A-A′,which shows the forward scanning direction. FIG. 7 is a cross-sectionalview taken along B-B′, which shows the backward scanning direction. Asshown in FIG. 5, an incident beam 121 having a rectangular, linear, orelliptical shape, which is formed with the convex lens 105 and deliveredto the non-single crystal semiconductor film 106, is scanned from aposition (1) to a position (2) on the non-single crystal semiconductorfilm as a rectangular, linear, elliptical beam 120 having the minor-axislength W and the major-axis length H. In this embodiment, the beam 120has a rectangular shape.

This beam is delivered to the non-single crystal semiconductor film 106as shown in FIGS. 6 and 7. The intensity distribution of the laser beamat the irradiation is not symmetric along a plane which passes throughthe center of the minor axis of the beam, which is perpendicular to thesubstrate, and which is parallel to the major axis of the beam evenafter adjusting the optical system in any way. In other words, theintensity distribution of the laser beam in the minor-axis direction isasymmetric along this plane. Therefore, the irradiation surface is notannealed in the same way in the forward and backward directions. Sincethe minor axis of the beam is as short as approximately 10 μm, it isdifficult to measure the intensity distribution of the beam in theminor-axis direction whether the intensity distribution is symmetric ornot along this plane. Further, it is much more difficult to adjust theintensity distribution of the laser beam in the minor-axis direction soas to be symmetric along this plane.

The characteristic of an electrical circuit follows a TFT having thelowest electron mobility among TFTs included in the electrical circuit.Therefore, when TFTs are manufactured using a semiconductor filmannealed by scanning the laser beam in directions shown in FIGS. 6 and 7alternately, the whole follows the characteristic of a TFT in theforward direction where the electron mobility is low.

Accordingly, the energy of the laser beam is preferably increased in theforward direction to such a degree that the electron mobility of the TFTbecomes as high as possible but the film is not ablated and so on, whilethe energy of the laser beam is preferably decreased in the backwarddirection to such a degree that the film is not ablated and so onbecause of the excessive energy.

That is to say, when the laser beam is scanned in the forward direction,the polarizing plate 102 (103) is rotated in the positive or negativedirection so as to increase the beam intensity of the incident beam 121.Meanwhile, when the laser beam is scanned in the backward direction, thepolarizing plate 102 (103) is rotated in the positive or negativedirection so as to decrease the intensity of the incident beam 121. As aresult, the whole surface of the non-single crystal semiconductor film106 can be crystallized homogenously with high energy.

Thus, in the case of annealing the non-single crystal semiconductor film106 by scanning the laser beam in the forward and backward directions,the energy of the laser beam is increased when the beam is scanned inthe direction where the mobility is lower, and the energy thereof isdecreased when the beam is scanned in the direction where the mobilityis higher. Thus, the variation of the crystallization can be suppressed.

The rectangular beam 120 being incident into the substrate 107 has asize of approximately 150 to 400 μm in the major-axis direction andapproximately 1 to 30 μm in the minor-axis direction. In the size of thebeam 120, the length in the major-axis direction may be determined inaccordance with the length in the minor-axis direction so that enoughenergy density can be provided. The upper limit of the length in theminor-axis direction is approximately 30 μm, while the lower limitthereof is approximately 1 μm. Out of this range, the large graincrystal having high characteristic is difficult to manufacture.Actually, in the case of a CW laser beam with 10 W which has a length ofapproximately 10 μm in the minor-axis direction, the length thereof inthe major-axis direction is approximately 400 μm. In the case of a CWlaser beam with 3 W which has a length of approximately 8 μm in theminor-axis direction, the length thereof in the major-axis direction isapproximately 150 μm.

FIG. 2 shows an example of another laser irradiation apparatus. Thislaser irradiation apparatus uses a diffractive optical element 110instead of the convex lens 105 in FIG. 1 in order to shape the beam intoa rectangular, linear, or elliptical beam.

That is to say, the laser irradiation apparatus comprises a laseroscillator 101, two polarizing plates 102 and 103, a mirror 104, and adiffractive optical element 110. The laser oscillator 101 is a CW laseroscillator, which is similar to that in FIG. 1. As a solid-state laser,a YAG laser, a YVO₄ laser, a YLF laser, a YAlO₃ laser, a GdVO₄ laser, aY₂O₃ laser, an alexandrite laser, or a Ti:sapphire laser can be used. Asa gas laser, an Ar laser, a Kr laser, or a CO₂ laser can be used. Apulsed laser can be used instead of the CW laser. As the pulsed laser, aYVO₄ laser, a GdVO₄ laser, a YAG laser, or the like which has arepetition frequency of 10 MHz or more can be used.

The diffractive optical element 110 is also referred to as a DOE or adiffractive optics. This element is to obtain the desired energy byusing the diffraction of light, and also serves as a condensing lens byforming many grooves on its surface. With this diffractive opticalelement 110, a laser beam emitted from the CW laser oscillator 101 whichhas Gaussian energy distribution can be shaped into a rectangular,linear, or elliptical beam having homogeneous energy distribution. Theother elements except the diffractive optical element 110 are the sameas those in FIG. 1; therefore the description of the details is omitted.

The non-single crystal semiconductor film 106 can be crystallizedhomogeneously by scanning the laser beam back and forth while changingthe beam intensity to anneal the non-single crystal semiconductor film106 similarly to FIG. 1 with such a laser irradiation apparatus.

The present invention is not limited to the constitution of the aboveembodiment and can be modified appropriately within the scope of thepresent invention. For example, a beam splitter or an ND filter (neutraldensity filter) can be used instead of the polarizing plates and thelike as means for varying beam intensity to adjust the intensity of thelaser beam. In this case, when the beam splitter or the ND filter issensitive to the laser, the beam diameter of the laser beam may beexpanded with a beam expander to decrease the energy density. In thecase of using a laser oscillator in which the output of the laser beamcan be adjusted by changing the intensity of an excitation light sourceof the laser, the intensity of the excitation light source may bechanged.

Embodiment 2

This embodiment describes a process for manufacturing a thin filmtransistor (a TFT) using a laser annealing apparatus according to thepresent invention.

As shown in FIG. 10A, a base film 1001 is formed over a substrate 1000having an insulating surface. A glass substrate is used as the substrate1000 in this embodiment. As the substrate used here, a glass substratemade of barium borosilicate glass or alumino borosilicate glass, aquartz substrate, a ceramic substrate, a stainless steel substrate, orthe like can be used. Although a substrate made from a flexible materialtypified by plastic, acrylic, or the like generally tends to be inferiorto the above substrates in point of the resistance against the heat, thesubstrate made of the flexible material can be used when the substratecan resist the heat generated in the manufacturing process.

The base film 1001 is provided in order to prevent an alkali-earth metalor an alkali metal such as Na included in the substrate 1000 fromdiffusing into the semiconductor film. The alkali-earth metal or thealkali metal causes an adverse effect on the characteristic of thesemiconductor element when the metal is in the semiconductor. Therefore,the base film is formed with an insulating material such as siliconoxide, silicon nitride, or silicon nitride oxide, which can suppress thediffusion of the alkali-earth metal and alkali metal into thesemiconductor. Further, the base film 1001 may have either asingle-layer structure or a multilayer structure. In the presentembodiment, a silicon nitride oxide film is formed in thickness from 10to 400 nm by a plasma CVD (Chemical Vapor Deposition) method.

When the substrate 1000 is a substrate including even a small amount ofthe alkali metal or the alkali-earth metal such as a glass substrate ora plastic substrate, it is effective to provide the base film in orderto prevent the diffusion of the impurity. When a substrate such as aquartz substrate is used which hardly diffuses the impurity, the basefilm 1001 is not always necessary to be provided.

Next, an amorphous semiconductor film 1002 is formed over the base film1001 in thickness from 25 to 100 nm (preferably from 30 to 60 nm) by aknown method (a sputtering method, an LPCVD method, a plasma CVD method,or the like). The amorphous semiconductor film 1002 may be formed withsilicon, silicon germanium, or the like, and silicon is used in thisembodiment. When silicon germanium is used, it is preferable that theconcentration of germanium be in the range of approximately 0.01 to 4.5atomic %.

Next, as shown in FIG. 10B, the amorphous semiconductor film 1002 isirradiated with a laser beam using a laser annealing apparatus of thepresent invention so that the semiconductor film is crystallized. Inthis embodiment, the irradiation is conducted with a convex lens 1005and a Nd:YVO₄ laser that provides 10 W at the second harmonic with thespatial profile of TEM₀₀ mode (single transverse mode). Not only thislaser but also another laser can be used. As a continuous wave (CW)solid-state laser, a YAG laser, a YLF laser, a YAlO₃ laser, a GdVO₄laser, a Y₂O₃ laser, an alexandrite laser, or a Ti:sapphire laser isgiven. As a CW gas laser, an Ar laser, a Kr laser, or a CO₂ laser isgiven. Further, a YVO₄ laser, a GdVO₄ laser, a YAG laser, and the likewhich have a pulse repetition frequency of 10 MHz or more can also beused.

By annealing the semiconductor film homogeneously in all the scanningdirections, the characteristic of an electronic appliance can bestabilized in a good condition. However, when a TFT is manufacturedusing a semiconductor film annealed alternately in directions shown inFIGS. 6 and 7, the crystallization state of the semiconductor film inthe TFT varies according to the forward and backward directions. Thiscauses significant effect on the characteristic of the electronicappliance.

In this embodiment, the semiconductor film is annealed homogeneously inall the scanning directions by using a polarizing plate as means forvarying beam intensity. Here, the beam intensity may be adjusted in suchaway that a layout is inputted to a control device for controlling amotor which moves the polarizing plate such as a computer in advance,the control device sends signals for controlling the motor in accordancewith the inputted layout, and the motor which has received the signalsmoves the polarizing plate.

Moreover, by using the slit, a low-intensity part of the laser beam canbe blocked; therefore a linear, elliptical, or rectangular laser beamhaving a predetermined intensity or more can be delivered.

After that, as shown in FIG. 10C, a crystalline semiconductor film 1003formed by the laser irradiation is patterned to form an island-shapedsemiconductor film 1006. Then, a gate insulating film 1007 is formed soas to cover the island-shaped semiconductor film 1006. The gateinsulating film 1007 can be formed with silicon oxide, silicon nitride,silicon nitride oxide, or the like by the plasma CVD method or thesputtering method. In this embodiment, a silicon nitride oxide film isformed in 115 nm thick by the plasma CVD method.

Next, a conductive film is formed over the gate insulating film 1007 andpatterned to form a gate electrode 1008. After that, a source region1009, a drain region 1010, an LDD region 1011, and the like are formedby adding an impurity imparting n-type or p-type conductivity to theisland-shaped semiconductor film 1006 using the gate electrode 1008 or apatterned resist as a mask. According to the above step, N-channel TFTs1012 and 1014 and a P-channel TFT 1013 can be formed over the samesubstrate.

Subsequently, as shown in FIG. 10D, an insulating film 1015 is formed asa protecting film for those TFTs. The insulating film 1015 is formedwith silicon nitride or silicon nitride oxide in thickness from 100 to200 nm in a single-layer or multilayer structure. In this embodiment, asilicon oxynitride film is formed in 100 nm thick by the plasma CVDmethod. With the insulating film 1015, it is possible to obtain ablocking effect for preventing the intrusion of oxygen and moisture inthe air and various ionic impurities.

Next, an insulating film 1016 is formed. In this embodiment, an organicresin film such as polyimide, polyamide, BCB (benzocyclobutene),acrylic, or siloxane, a TOF film, an inorganic interlayer insulatingfilm (an insulating film including silicon such as silicon nitride orsilicon oxide), a low-k (low dielectric constant) material, or the likecan be used. Siloxane is a material which has a bond of silicon andoxygen expressed with —Si—O—Si— (siloxane bond) as a basic unit and hasa constitution in which silicon is combined with fluorine, aliphatichydrocarbon, aromatic hydrocarbon, or the like. Since the insulatingfilm 1016 is formed mainly for the purpose of relaxing and flatteningthe unevenness due to the TFTs formed over the glass substrate, a filmbeing superior in flatness is preferable.

Moreover, an insulating film and an organic insulating film arepatterned by a photolithography method to form contact holes that reachthe impurity regions.

Next, a conductive film is formed with a conductive material, and awiring 1017 is formed by patterning the conductive film. After that,when an insulating film 1018 is formed as a protective film, asemiconductor device shown in FIG. 10D is completed. It is to be notedthat the method for manufacturing a semiconductor device using the laserannealing method of the present invention is not limited to the abovemethod for manufacturing a TFT.

Before the laser crystallization step, a crystallization step using acatalyst element may be provided. As the catalyst element, nickel (Ni),germanium (Ge), iron (Fe), palladium (Pd), tin (Sn), lead (Pb), cobalt(Co), platinum (Pt), copper (Cu), or gold (Au) can be used.

When the crystallization step by the laser irradiation is performedafter the crystallization step using the catalyst element, it ispossible to enhance the crystallinity of the semiconductor film furtherand to suppress the roughness of the surface of the semiconductor filmafter the laser crystallization, compared to the case in which thesemiconductor film is crystallized only by the laser irradiation.Therefore, the variation of the characteristics of the semiconductorelement to be formed afterward typified by a TFT can be more suppressedand the off-current can be suppressed.

It is to be noted that the crystallization may be performed in such away that after the catalyst element is added, the heat treatment isperformed in order to promote the crystallization, and then the laserirradiation is conducted. Alternatively, the heat treatment may beomitted. Further, the laser process may be performed after the heattreatment while keeping the temperature.

Although the present embodiment shows an example in which the laserirradiation method of the present invention is used to crystallize thesemiconductor film, the laser irradiation method may be applied toactivate the impurity element doped in the semiconductor film. Moreover,the method for manufacturing a semiconductor device of the presentinvention can be applied to a method for manufacturing an integratedcircuit and a semiconductor display device.

A transistor used for a functional circuit such as a driver or a CPU(central processing unit) preferably has an LDD structure or a structurein which the LDD overlaps the gate electrode. It is desirable tominiature the transistor in order to increase the speed. Since thetransistor completed according to the present embodiment has the LDDstructure, the transistor is preferably used for a driver circuitrequiring high-speed operation.

Embodiment 3

According to the present invention, various electronic appliances can becompleted using thin film transistors. Specific examples are describedwith reference to FIGS. 11A to 11C.

FIG. 11A shows a display device including a case 1101, a supportingstand 1102, a display portion 1103, speaker portions 1104, a video inputterminal 1105, and the like. This display device is manufactured byapplying the thin film transistors, which have been formed by themanufacturing method shown in FIGS. 10A to 10D, in the display portion1103. It is to be noted that the display device includes a liquidcrystal display device and a light-emitting device, specifically allkinds of display devices for displaying information for a computer,television reception, advertisement, and so on.

FIG. 11B shows a computer including a case 1111, a display portion 1112,a keyboard 1113, an external connection port 1114, a pointing mouse1115, and the like. By applying the manufacturing method shown in FIGS.10A to 10D, the display portion 1112 and other circuits can bemanufactured. Further, the present invention can be applied to anothersemiconductor device such as a CPU and a memory inside the main body.

FIG. 11C shows a mobile phone, which is a typical example of mobileterminals. This mobile phone includes a case 1121, operation keys 1122,a display portion 1123, and the like. In addition to the mobile phone,since electronic appliances such as a PDA (personal digital assistant),a digital camera, and a small game machine are mobile terminals, theirdisplay screens are small. Therefore, by forming the functional circuitssuch as a CPU and a memory with the small transistors shown in FIGS. 10Ato 10D, smaller and more light-weight appliances can be achieved.

The thin film transistor manufactured in this embodiment can be used asan ID chip. For example, by the manufacturing method shown in FIGS. 10Ato 10D, the transistors can be applied as an integrated circuit and amemory in the ID chip or as an ID tag. When the transistors are used asthe memory, a circulation process of a product and a process of aproduction step such as a production area, a producer, a manufacturingdate, and a process method can be recorded. It becomes easy forwholesalers, retailers, and consumers to know these information.

Further, in the case of using the TFTs as an ID tag with aradio-frequency function mounted, settlement of products and inventorywork can be simplified by using the ID tags instead of conventionalbarcodes. Moreover, forgetting the settlement and shoplifting can beprevented by inputting that the settlement has been done into the ID tagat the settlement, and checking the ID tag whether the settlement hasbeen done by checking means provided at the exit.

As thus described, the semiconductor device manufactured according tothe present invention can be applied in a wide range, and thesemiconductor device manufactured according to the present invention canbe applied to various electronic appliances in various fields.

Explanation of Reference

1: LASER OSICILLATOR, 2: CYLINDRICAL LENS ARRAY, 3: CYLINDRICAL LENSARRAY, 4: CYLINDRICAL LENS ARRAY, 5: CYLINDRICAL LENS ARRAY, 6:CYLINDRICAL LENS ARRAY, 7: MIRROR, 8: DOUBLET CYLINDRICAL LENS, 9:IRRADIATION SURFACE, 11: LASER OSCILLATOR, 12: MIRROR, 13: CONVEX LENS,14: BEAM, 15: IRRADIATION SURFACE, 101: LASER OSCILLATOR, 102:POLARIZING PLATE, 103: POLARIZING PLATE, 104: MIRROR, 105: CONVEX LENS,106: NON-SINGLE CRYSTAL SEMICONDUCTOR FILM, 107: SUBSTRATE, 108: X-AXISSTAGE, 109: Y-AXIS STAGE, 110: DIFFRACTIVE OPTICAL ELEMENT, 120: BEAM,121: INCIDENT BEAM, 1000: SUBSTRATE, 1001: BASE FILM, 1002: AMORPHOUSSEMICONDUCTOR FILM, 1003: CRYSTALLINE SEMICONDUCTOR FILM, 1005: CONVEXLENS, 1006: SEMICONDUCTOR FILM, 1007: GATE INSULATING FILM, 1008: GATEELECTRODE, 1009: SOURCE REGION, 1010: DRAIN REGION, 1011: LDD REGION,1012: N-CHANNEL TFT, 1013: P-CHANNEL TFT, 1014: N-CHANNEL TFT, 1015:INSULATING FILM, 1016: INSULATING FILM, 1017: WIRING 1018: INSULATINGFILM, 1101: CASE, 1102: SUPPORTING STAND, 1103: DISPLAY PORTION, 1104:SPEAKER PORTIONS, 1105: VIDEO INPUT TERMINAL, 1111: CASE, 1112: DISPLAYPORTION, 1113: KEYBOARD, 1114: EXTERNAL CONNECTION PORT, 1115: POINTINGMOUSE, 1121: CASE, 1122: OPERATION KEYS, 1123: DISPLAY PORTION

1. A laser irradiation method comprising: delivering a laser beam to a semiconductor layer; scanning the semiconductor layer to a first direction with the laser beam in a first intensity; and scanning the semiconductor layer to a second direction with the laser beam in a second intensity, wherein the first intensity is larger than the second intensity, wherein the first direction is a forward direction, and wherein the second direction is a backward direction.
 2. The laser irradiation method according to claim 1, wherein the laser beam is delivered obliquely to the semiconductor layer.
 3. The laser irradiation method according to claim 1, wherein the semiconductor layer moves to a direction reverse to the first direction, when the semiconductor layer is scanned to a first direction.
 4. The laser irradiation method according to claim 1, wherein the semiconductor layer moves to a direction reverse to the second direction, when the semiconductor layer is scanned to a second direction.
 5. A laser irradiation method comprising: emitting a first laser beam; changing the first laser beam into a second laser beam through means for varying beam intensity which can vary beam intensity; changing the second laser beam into a third laser beam; delivering the third laser beam to an irradiation surface; and scanning the irradiation surface with the third laser beam, wherein the third laser beam is scanned in a forward direction in a first period, wherein the third laser beam is scanned in a backward direction in a second period, and wherein the beam intensity of the second laser beam in the first period is larger than that in the second period.
 6. The laser irradiation method according to claim 5, wherein the third laser beam is delivered obliquely to the irradiation surface.
 7. The laser irradiation method according to claim 5, wherein the first laser beam is emitted from a laser oscillator.
 8. The laser irradiation method according to claim 5, wherein the means for varying beam intensity comprises at least one of polarizing plates and an ND filter.
 9. The laser irradiation method according to claim 5, wherein the second laser beam is changed into a third laser beam through at least one of a convex lens and a diffractive optical element.
 10. The laser irradiation method according to claim 5, wherein the irradiation surface moves to a direction reverse to a scanning direction, when the irradiation surface is scanned with the third laser beam.
 11. The laser irradiation method according to claim 5, wherein the irradiation surface is a surface of a semiconductor layer.
 12. A laser irradiation apparatus comprising: a laser oscillator; means for varying beam intensity; and a convex lens; wherein a laser beam is incident into an irradiation surface, wherein the irradiation surface is scanned with the laser beam in a forward direction in a first period, wherein the irradiation surface is scanned with the laser beam in a backward direction in a second period, and wherein beam intensity is varied between the first period and the second period by the means for varying beam intensity.
 13. The laser irradiation apparatus according to claim 12, wherein the laser beam passed through the convex lens has a rectangular, linear, or elliptical shape on the irradiation surface.
 14. The laser irradiation apparatus according to claim 12, wherein the means for varying beam intensity comprises at least one of a polarizing plate and an ND filter.
 15. The laser irradiation apparatus according to claim 14, wherein the number of the polarizing plates is more than one.
 16. The laser irradiation apparatus according to claim 12, wherein the laser oscillator is a continuous wave solid-state laser, gas laser, or metal laser or a pulsed solid-state laser, gas laser, or metal laser.
 17. The laser irradiation apparatus according to claim 12, wherein the laser oscillator is one selected from the group consisting of continuous wave or pulsed YAG laser, YVO₄ laser, YLF laser, YAIO₃ laser, GdVO₄ laser, Y₂O₃ laser, glass laser, ruby laser, alexandrite laser, and Ti:sapphire laser.
 18. The laser irradiation apparatus according to claim 12, wherein the laser oscillator is one selected from the group consisting of an Ar laser, a Kr laser, and a CO₂ laser.
 19. The laser irradiation apparatus according to claim 12, wherein the laser oscillator is one selected from the group consisting of a YVO₄ laser, a GdVO₄ laser, and a YAG laser which have a repetition frequency of 10 MHz or more.
 20. The laser irradiation apparatus according to claim 12, wherein the laser beam emitted from the laser oscillator is converted into a harmonic by a non-linear optical element.
 21. A laser irradiation apparatus comprising: a laser oscillator; means for varying beam intensity; and a diffractive optical element; wherein a laser beam is incident into an irradiation surface, wherein the irradiation surface is scanned with the laser beam in a forward direction in a first period, wherein the irradiation surface is scanned with the laser beam in a backward direction in a second period, and wherein beam intensity is varied between the first period and the second period by the means for varying beam intensity.
 22. The laser irradiation apparatus according to claim 21, wherein the laser beam passed through the diffractive optical element has a rectangular, linear, or elliptical shape on the irradiation surface.
 23. The laser irradiation apparatus according to claim 21, wherein the means for varying beam intensity comprises at least one of a polarizing plate and an ND filter.
 24. The laser irradiation apparatus according to claim 23, wherein the number of the polarizing plates is more than one.
 25. The laser irradiation apparatus according to claim 21, wherein the laser oscillator is a continuous wave solid-state laser, gas laser, or metal laser or a pulsed solid-state laser, gas laser, or metal laser.
 26. The laser irradiation apparatus according to claim 21, wherein the laser oscillator is one selected from the group consisting of continuous wave or pulsed YAG laser, YVO₄ laser, YLF laser, YAIO₃ laser, GdVO₄ laser, Y₂O₃ laser, glass laser, ruby laser, alexandrite laser, and Ti:sapphire laser.
 27. The laser irradiation apparatus according to claim 21, wherein the laser oscillator is one selected from the group consisting of an Ar laser, a Kr laser, and a CO₂ laser.
 28. The laser irradiation apparatus according to claim 21, wherein the laser oscillator is one selected from the group consisting of a YVO₄ laser, a GdVO₄ laser, and a YAG laser which have a repetition frequency of 10 MHz or more.
 29. The laser irradiation apparatus according to claim 21, wherein the laser beam emitted from the laser oscillator is converted into a harmonic by a non-linear optical element.
 30. A laser irradiation apparatus comprising: a laser oscillator; means for varying beam intensity; and a convex lens; wherein a laser beam is incident obliquely into an irradiation surface, wherein the irradiation surface is scanned with the laser beam in a forward direction in a first period, wherein the irradiation surface is scanned with the laser beam in a backward direction in a second period, and wherein beam intensity is varied between the first period and the second period by the means for varying beam intensity.
 31. The laser irradiation apparatus according to claim 30, wherein the laser beam passed through the convex lens has a rectangular, linear, or elliptical shape on the irradiation surface.
 32. The laser irradiation apparatus according to claim 30, wherein the means for varying beam intensity comprises at least one of a polarizing plate and an ND filter.
 33. The laser irradiation apparatus according to claim 32, wherein the number of the polarizing plates is more than one.
 34. The laser irradiation apparatus according to claim 30, wherein the laser oscillator is a continuous wave solid-state laser, gas laser, or metal laser or a pulsed solid-state laser, gas laser, or metal laser.
 35. The laser irradiation apparatus according to claim 30, wherein the laser oscillator is one selected from the group consisting of continuous wave or pulsed YAG laser, YVO₄ laser, YLF laser, YAIO₃ laser, GdVO₄ laser, Y₂O₃ laser, glass laser, ruby laser, alexandrite laser, and Ti:sapphire laser.
 36. The laser irradiation apparatus according to claim 30, wherein the laser oscillator is one selected from the group consisting of an Ar laser, a Kr laser, and a CO₂ laser.
 37. The laser irradiation apparatus according to claim 30, wherein the laser oscillator is one selected from the group consisting of a YVO₄ laser, a GdVO₄ laser, and a YAG laser which have a repetition frequency of 10 MHz or more.
 38. The laser irradiation apparatus according to claim 30, wherein the laser beam emitted from the laser oscillator is converted into a harmonic by a non-linear optical element.
 39. A laser irradiation apparatus comprising: a laser oscillator; means for varying beam intensity; and a diffractive optical element; wherein a laser beam is incident obliquely into an irradiation surface, wherein the irradiation surface is scanned with the laser beam in a forward direction in a first period, wherein the irradiation surface is scanned with the laser beam in a backward direction in a second period, and wherein beam intensity is varied between the first period and the second period by the means for varying beam intensity.
 40. The laser irradiation apparatus according to claim 39, wherein the laser beam passed through the diffractive optical element has a rectangular, linear, or elliptical shape on the irradiation surface.
 41. The laser irradiation apparatus according to claim 39, wherein the means for varying beam intensity comprises at least one of a polarizing plate and an ND filter.
 42. The laser irradiation apparatus according to claim 41, wherein the number of the polarizing plates is more than one.
 43. The laser irradiation apparatus according to claim 39, wherein the laser oscillator is a continuous wave solid-state laser, gas laser, or metal laser or a pulsed solid-state laser, gas laser, or metal laser.
 44. The laser irradiation apparatus according to claim 39, wherein the laser oscillator is one selected from the group consisting of continuous wave or pulsed YAG laser, YVO₄ laser, YLF laser, YAlO₃ laser, GdVO₄ laser, Y₂O₃ laser, glass laser, ruby laser, alexandrite laser, and Ti:sapphire laser.
 45. The laser irradiation apparatus according to claim 39, wherein the laser oscillator is one selected from the group consisting of an Ar laser, a Kr laser, and a CO₂ laser.
 46. The laser irradiation apparatus according to claim 39, wherein the laser oscillator is one selected from the group consisting of a YVO₄ laser, a GdVO₄ laser, and a YAG laser which have a repetition frequency of 10 MHz or more.
 47. The laser irradiation apparatus according to claim 39, wherein the laser beam emitted from the laser oscillator is converted into a harmonic by a non-linear optical element. 